On an individual level, I'd brutally kill the man too. However, when it comes to the law, I think you are right. Vigilante justice should not be tolerated. The mother should have a suspended sentence though because of the circumstances.While it is obviously wrong to rape a child, vigilante justice is not a good solution either.
Let's imagine that this is not rape of a child... someone stealing something from the store. And they're innocent. Would you justify carving them up?
And yes, I think the fool that raped this child DOES deserve hell.... but he deserves hell in prison. This is my opinion.